[RISCV] Combine concat_vectors of loads into strided loads

If we're concatenating several smaller loads separated by a stride, we
can try and increase the element size and perform a strided load.
For example:

```
concat_vectors (load v4i8, p+0), (load v4i8, p+n), (load v4i8, p+n*2), (load v4i8, p+n*3)
=>
vlse32 p, stride=n, VL=4
```

This pattern can be produced by the SLP vectorizer.

A special case is when the stride is exactly equal to the width of the
vector, in which case it can be converted into a single consecutive
vector load. For example:

```
concat_vectors (load v4i8, p), (load v4i8, p+4), (load v4i8, p+8), (load v4i8, p+12)
=>
vle8 p, VL=16
```

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D147713